Recurrent Aphthous Stomatitis (Canker Sore): This is the most common cause of a sore on the gum, characterized by small, painful ulcers that can appear on the gums, tongue, or inside the cheeks. The exact cause is unknown, but it's thought to be related to stress, minor mouth injury, or certain foods.

Gingivitis or Periodontal Abscess: Inflammation of the gums (gingivitis) or a localized collection of pus (periodontal abscess) in the gum tissue can cause soreness and pain. These conditions are often associated with poor oral hygiene.
Dental Abscess: A bacterial infection that causes a pocket of pus to form in the tooth or gum, leading to severe pain and swelling.
Oral Trauma: Accidental bites, burns from hot food or drinks, or other injuries can cause sores on the gums.
Herpes Simplex Virus (HSV) Infection: Primary or recurrent HSV infection can cause painful sores on the gums, often accompanied by fever and swollen lymph nodes.

Squamous Cell Carcinoma: Although rare, oral cancer can present as a non-healing sore on the gum. It's crucial to consider this diagnosis, especially in patients with risk factors like tobacco use or a history of radiation therapy.
Tuberculosis (TB): Oral TB can manifest as a sore on the gum, particularly in immunocompromised individuals or those from endemic areas.
Syphilis: Secondary syphilis can cause mucous patches or ulcers on the gums, which can be painless and may go unnoticed.

Eosinophilic Ulcer: A rare condition characterized by a painful ulcer on the gum, often associated with eosinophilia (an increase in eosinophils, a type of white blood cell).
Pyostomatitis Vegetans: A rare inflammatory condition that causes multiple small ulcers and vegetating lesions on the gums and other oral mucosal surfaces.
Mucous Membrane Pemphigoid: A group of autoimmune diseases that can cause blistering lesions on the gums and other mucous membranes.